In the field of graphic arts, printing of a press plate has been conducted using a set of color separation films prepared from a color original by using lith type films. In general, a color proof is produced from color separation films, before printing (actual printing work), in order to check for errors in the color separation step or necessity of color compensation. The color proof is desired to realize an enough high resolution to permit high reproduction of a middle tone image, a high step stability and the like. In addition, in order to obtain a color proof similar to actual printed products, it is preferred to use, as materials for the color proof, those which are used for actual printed products—for example, regular printing papers as substrates, and pigments as coloring materials. Also, as a method for producing a color proof, a dry method is more desired which does not use any developing solution.
With the recent diffusion of electronic systems in the pre-printing step (pre-press field), there has been developed, as a dry method for producing a color proof, a recording system wherein a color proof is directly produced from digital signals. Such electronic system intends to produce, particularly, color proofs with a high image quality and, in general, it reproduces a half-tone dot image of 150 lines/inch or more. In order to record a high-quality proof from digital signals, a laser light which can be modulated by digital signals and permits to finely focus the recording light is used as a recording head. Thus, it has become necessary to develop an image-forming material which shows a high recording sensitivity to a laser light and an enough high resolving power to permit reproduction of highly fine half-tone dots.
As an image-forming material to be used for a transfer image-forming method using a laser light, there has been known a heat-melting transfer sheet comprising a support having provided thereon a light-to-heat conversion layer capable of absorbing a laser light to generate heat and an image-forming layer wherein a pigment is dispersed in a heat-meltable binder such as wax or a binder, in this order (Japanese Patent Laid-Open No. 58045/1993). In the image-forming method using such image-forming materials, heat generated in the laser light-irradiated area of the light-to-heat conversion layer melts the image-forming layer of the corresponding area, and the molten portion of the image-receiving layer is transferred to an image-receiving sheet disposed, in layers, on the transfer sheet, thus a transfer image being formed on the image-receiving sheet.
Also, Japanese Patent 219052/1994 discloses a heat transfer sheet which comprises a support having provided thereon a light-to-heat conversion layer containing a light-heat converting substance, an extremely thin (0.03 to 0.3 μm) heat releasable layer and an image-forming layer containing a coloring material in this order. In this thermal transfer sheet, the binding force between the image-forming layer and the light-to-heat conversion layer which are bound to each other by the heat releasable layer provided therebetween is reduced by irradiation of a laser light, and a highly fine image is formed on an image-receiving layer disposed, in layers, on the thermal transfer sheet. The aforesaid image-forming method using the thermal transfer sheet utilizes so-called “abrasion”. Specifically, the method utilizes the phenomenon that the heat releasable layer is partly decomposed in the area which has been irradiated with a laser light, and is gasified, and hence bonding force between the image-forming layer and the light-to-heat conversion layer is so weakened in the laser-irradiated area that the image-forming layer in the area is transferred to the image-receiving sheet superimposed thereon.
These image-forming methods have the advantages that regular printing paper having provided thereon an image-receiving layer (adhesive layer) can be used as an image-receiving sheet material, and that a multi-color image can easily be obtained by successively transferring images with a different color onto the image-receiving sheet. In particular, the image-forming method utilizing abrasion has the advantage that a highly fine image can be obtained with ease, and is useful for producing a color proof (DDCP: Direct Digital Color Proof)or a highly fine mask image.
With the progress of DTP environment, there is an increasing need for proof by DDCP system among users of CTP (Computer To Plate) instead of the proof system of a conventional proof or of an analogue technique, because an intermediate film-producing step can be eliminated. In recent years, there has been desired a large-sized DDCP having a high quality, a high stability and an excellent printing compatibility.
The laser thermal transfer method permits printing with a high resolution, and there have conventionally been such systems as (1) laser sublimation system, (2) laser abrasion system, and (3) laser melting system. Howver, all of them involve the problem that form of recorded half-tone dots is not sharp. The laser sublimation system (1) involves the problem that, since it uses dyes as coloring materials, similarity to printed products is not enough and, in addition, outline of half-tone dots becomes blurred due to sublimation properties of the coloring materials, thus resolution not being sufficiently high. On the other hand, the laser abrasion system shows a good similarity to printed products since it uses pigments as coloring materials but, since the coloring materials are scattered in this system, outline of half-tone dots become blurred similarly with the sublimation system, thus resolution not being sufficiently high. Further, the laser melting system (3) involves the problem that clear outline cannot be formed due to flow of the molten substance.
Also, the conventionally employed thermal transfer sheets are limited to so-called process color technique using the four colors of yellow, magenta, cyan and black, thus the range of reproducible hues being limited.
The subject of the invention is to solve the aforesaid problems with conventional art, and to attain the following objects. That is, an object of the invention is to provide a multi-color image-forming material which can reproduce an expanded range of hues, and a multi-color image-forming method using the same. Further, another object of the invention is to provide a multi-color image-forming material capable of providing a large-sized DDCP having a high quality, a high stability and an excellent printing compatibility, and a multi-color image-forming method using the same. Also, a further object of the invention is to provide a multi-color image-forming material which can form an image with a good image quality and a stable transfer density even when subjected to laser recording with a high energy by a laser light of multi-beams, and a multi-color image-forming method using the same.